241 lines
8.8 KiB
ReStructuredText
241 lines
8.8 KiB
ReStructuredText
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Developing Cipher Algorithms
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============================
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Registering And Unregistering Transformation
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--------------------------------------------
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There are three distinct types of registration functions in the Crypto
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API. One is used to register a generic cryptographic transformation,
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while the other two are specific to HASH transformations and
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COMPRESSion. We will discuss the latter two in a separate chapter, here
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we will only look at the generic ones.
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Before discussing the register functions, the data structure to be
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filled with each, struct crypto_alg, must be considered -- see below
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for a description of this data structure.
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The generic registration functions can be found in
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include/linux/crypto.h and their definition can be seen below. The
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former function registers a single transformation, while the latter
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works on an array of transformation descriptions. The latter is useful
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when registering transformations in bulk, for example when a driver
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implements multiple transformations.
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::
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int crypto_register_alg(struct crypto_alg *alg);
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int crypto_register_algs(struct crypto_alg *algs, int count);
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The counterparts to those functions are listed below.
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::
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void crypto_unregister_alg(struct crypto_alg *alg);
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void crypto_unregister_algs(struct crypto_alg *algs, int count);
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The registration functions return 0 on success, or a negative errno
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value on failure. crypto_register_algs() succeeds only if it
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successfully registered all the given algorithms; if it fails partway
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through, then any changes are rolled back.
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The unregistration functions always succeed, so they don't have a
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return value. Don't try to unregister algorithms that aren't
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currently registered.
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Single-Block Symmetric Ciphers [CIPHER]
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---------------------------------------
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Example of transformations: aes, serpent, ...
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This section describes the simplest of all transformation
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implementations, that being the CIPHER type used for symmetric ciphers.
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The CIPHER type is used for transformations which operate on exactly one
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block at a time and there are no dependencies between blocks at all.
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Registration specifics
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~~~~~~~~~~~~~~~~~~~~~~
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The registration of [CIPHER] algorithm is specific in that struct
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crypto_alg field .cra_type is empty. The .cra_u.cipher has to be
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filled in with proper callbacks to implement this transformation.
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See struct cipher_alg below.
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Cipher Definition With struct cipher_alg
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Struct cipher_alg defines a single block cipher.
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Here are schematics of how these functions are called when operated from
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other part of the kernel. Note that the .cia_setkey() call might happen
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before or after any of these schematics happen, but must not happen
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during any of these are in-flight.
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::
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KEY ---. PLAINTEXT ---.
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v v
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.cia_setkey() -> .cia_encrypt()
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'-----> CIPHERTEXT
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Please note that a pattern where .cia_setkey() is called multiple times
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is also valid:
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::
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KEY1 --. PLAINTEXT1 --. KEY2 --. PLAINTEXT2 --.
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v v v v
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.cia_setkey() -> .cia_encrypt() -> .cia_setkey() -> .cia_encrypt()
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'---> CIPHERTEXT1 '---> CIPHERTEXT2
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Multi-Block Ciphers
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-------------------
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Example of transformations: cbc(aes), chacha20, ...
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This section describes the multi-block cipher transformation
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implementations. The multi-block ciphers are used for transformations
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which operate on scatterlists of data supplied to the transformation
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functions. They output the result into a scatterlist of data as well.
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Registration Specifics
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~~~~~~~~~~~~~~~~~~~~~~
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The registration of multi-block cipher algorithms is one of the most
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standard procedures throughout the crypto API.
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Note, if a cipher implementation requires a proper alignment of data,
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the caller should use the functions of crypto_skcipher_alignmask() to
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identify a memory alignment mask. The kernel crypto API is able to
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process requests that are unaligned. This implies, however, additional
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overhead as the kernel crypto API needs to perform the realignment of
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the data which may imply moving of data.
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Cipher Definition With struct skcipher_alg
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Struct skcipher_alg defines a multi-block cipher, or more generally, a
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length-preserving symmetric cipher algorithm.
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Scatterlist handling
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~~~~~~~~~~~~~~~~~~~~
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Some drivers will want to use the Generic ScatterWalk in case the
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hardware needs to be fed separate chunks of the scatterlist which
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contains the plaintext and will contain the ciphertext. Please refer
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to the ScatterWalk interface offered by the Linux kernel scatter /
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gather list implementation.
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Hashing [HASH]
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--------------
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Example of transformations: crc32, md5, sha1, sha256,...
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Registering And Unregistering The Transformation
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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There are multiple ways to register a HASH transformation, depending on
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whether the transformation is synchronous [SHASH] or asynchronous
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[AHASH] and the amount of HASH transformations we are registering. You
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can find the prototypes defined in include/crypto/internal/hash.h:
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::
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int crypto_register_ahash(struct ahash_alg *alg);
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int crypto_register_shash(struct shash_alg *alg);
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int crypto_register_shashes(struct shash_alg *algs, int count);
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The respective counterparts for unregistering the HASH transformation
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are as follows:
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::
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void crypto_unregister_ahash(struct ahash_alg *alg);
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void crypto_unregister_shash(struct shash_alg *alg);
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void crypto_unregister_shashes(struct shash_alg *algs, int count);
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Cipher Definition With struct shash_alg and ahash_alg
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Here are schematics of how these functions are called when operated from
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other part of the kernel. Note that the .setkey() call might happen
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before or after any of these schematics happen, but must not happen
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during any of these are in-flight. Please note that calling .init()
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followed immediately by .finish() is also a perfectly valid
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transformation.
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::
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I) DATA -----------.
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v
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.init() -> .update() -> .final() ! .update() might not be called
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^ | | at all in this scenario.
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'----' '---> HASH
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II) DATA -----------.-----------.
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v v
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.init() -> .update() -> .finup() ! .update() may not be called
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^ | | at all in this scenario.
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'----' '---> HASH
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III) DATA -----------.
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v
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.digest() ! The entire process is handled
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'---------------> HASH
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Here is a schematic of how the .export()/.import() functions are called
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when used from another part of the kernel.
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::
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KEY--. DATA--.
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v v ! .update() may not be called
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.setkey() -> .init() -> .update() -> .export() at all in this scenario.
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^ | |
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'-----' '--> PARTIAL_HASH
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----------- other transformations happen here -----------
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PARTIAL_HASH--. DATA1--.
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v v
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.import -> .update() -> .final() ! .update() may not be called
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^ | | at all in this scenario.
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'----' '--> HASH1
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PARTIAL_HASH--. DATA2-.
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v v
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.import -> .finup()
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'---------------> HASH2
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Note that it is perfectly legal to "abandon" a request object:
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- call .init() and then (as many times) .update()
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- _not_ call any of .final(), .finup() or .export() at any point in future
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In other words implementations should mind the resource allocation and clean-up.
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No resources related to request objects should remain allocated after a call
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to .init() or .update(), since there might be no chance to free them.
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Specifics Of Asynchronous HASH Transformation
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Some of the drivers will want to use the Generic ScatterWalk in case the
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implementation needs to be fed separate chunks of the scatterlist which
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contains the input data. The buffer containing the resulting hash will
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always be properly aligned to .cra_alignmask so there is no need to
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worry about this.
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